Isolators in lumped constant systems



Nov. 21, 1961 H. SEIDEL ISOLATORS IN LUMPED CONSTANT SYSTEMS Filed Nov. 17, 1958 15 f FIG. 6'

22 I 1 E2 2. 22 2a o: FIG. 7

2 Sheets-Sheet 1 FIG. 3

FIG. 8

INVENTOR ATTORNEY FIG. 9

Nov. 21, 1961 H. SEIDEL ISOLATORS IN LUMPED CONSTANT SYSTEMS Filed Nov. 17, 1958 2 Sheets-Sheet 2 INVENTOYR H. S E IDE L A TTORNEV United States Patet 3,010,085 Patented Nov. 21, 1961 flice 3,010,085 ISOLATORS IN LUMPED CONSTANT SYSTEMS Harold Seidel, Fanwood, N.J., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Filed Nov. 17, 1958, Ser. No. 774,389 3 Claims. (Cl. 33324) This invention relates to nonreciprocal transmission circuits for electromagnetic wave energy and more particularly to isolators in lumped constant systems.

The use of materials having gyromagnetic properties to obtain both reciprocal and nonreciprocal effects in microwave transmission circuits is widely known and has found numerous and varied application in propagation structures of both the waveguide and the transmission line types. A rsum of the early work done using distributed circuit elements is contained in technical papers too numerous to mention. It has become apparent, however, that the need for nonreciprocal circuit elements is at least as great in the lower frequency ranges in which lumped circuit elements are normally used.

Included among the new transmission components that have found widespread use in the microwave art and which would be very useful in the low frequency lumped constant circuits is the so-called isolator. The isolator may be defined as a circuit element which is transparent to electromagnetic transmissions in one direction, designated the forward direction, whereas electromagnetic transmissions in the opposite or reverse direction are attenuated.

Included among the several types of isolators known to the prior art is the so-called resonant isolator. The resonant isolator operates by virtue of the absorption of power by the magnetically biased gyromagnetic material from a circularly polarized radio frequency magnetic field. Nonreciprocal attenuation effects are produced as a consequence of the ability of the magnetically polarized gyromagnetic material to distinguish between oppositely rotating magnetic fields. Thus, energy is only absorbed for one sense of polarization.

It is therefore an object of this invention to induce circularly polarized magnetic radio frequency fields in a gyromagnetic material in a lumped constant electrical system.

It is a further object of this invention that the sense of said circularly polarized magnetic fields be reversed for opposite directions of transmission through said system.

In accordance with the invention, a circularly polarized wave is induced in a resonantly biased element of gyromagnetic material by simultaneously controlling the relative time phase and space phase of a pair of intersecting radio frequency magnetic field components. Reciprocal and nonreciprocal coupling means are provided between the input and output meshes of a lumped constant network. In accordance with the invention, the couplings and the associated components of the network are adjusted to establish a pair of radio frequency magnetic field componentsthat have a relative time phase B, and a relative space phase a, such that fi=[(2n+l)1ra], where n is an integer. The relative time phase of one of the induced fields is further adjusted to be either lagging or leading with respect to the other of said fields depending upon the direction .of propagation through the network. As a consequence, the resulting magnetic field is a circularly polarized field having a sense of rotation which depends upon the direction of transmission through the network.

Nonreciprocal coupling between the meshes is provided by an element of resonantly biased gyromagnetic material disposed in the region of the rotating magnetic field. The gyromagnetic material is loosely coupled to the network. Because of the loose coupling the energy exchange process between the network and the gyromagnetic material may be thought of in terms of a perturbation phenomenon.

Isolator effects occur as a result of the nonreciprocal coupling produced by the gyromagnetic material in response to the action of'the circularly polarized magnetic fields.

In a first embodiment of the invention, both the reciprocal and the nonreciprocal coupling between the input and output circuits utilize a common circuit component. In this embodiment, the mutual coupling between the input and output circuits is provided by an inductive coil which is also part of the nonreciprocal coupling mechanism. The latter comprises a pair of coils and an element of resonantly biased gyromagnetic material. The rotating field effect is obtained by winding the pair of coils on the gyromagnetic material so that their axes are normal to each other, and by energizing the coils 90 out of time phase.

In a second embodiment of the invention, the reciprocal and nonreciprocal couplings are independently performed by separate transformers. As in the first embodiment, the time and space relationships of the magnetic field components in the region of the gyromagnetic material are such as to produce a circularly polarized magnetic field having a sense of rotation which varies as a function of the direction of transmission through the system.

These and other objects and advantages, the nature of the present invention, and its various features, will appear more fully upon consideration of the various illustrative embodiments now to be described in detail in connection with the accompanying drawings, in which:

FIG. 1 is a combined schematic and perspective view of the first embodiment of the invention;

FIG. 2 is a schematic diagram of the isolator illustrated in FIG. 1;

FIG. 3 shows, by way of illustration, the time relationships of the energizing currents for transmission in the forward direction;

FIG. 4 shows, by way of illustration, the space orientation of the polarizing direct-current field and the radio frequency field components for transmission in the forward direction;

FIG. 5 shows, by way of illustration, the time relationships of the energizing currents for transmission in the reverse direction;

FIG. 6 shows, by way of illustration, the space orientation of the polarizing direct-current field and the radio frequency field components for transmission in the reverse directions;

FIG. 7 is a schematic diagram of the second embodiment of the invention showing separate reciprocal and nonreciprocal coupling means;

FIG. 8 is a time' vector diagram of the energizing currents in the second embodiment of the invention for transmission in the forward and reverse directions; and

FIG. 9 is a circuit diagram of the equivalent circuit of the nonreciprocal coupling means showing the current relationships for transmission in the forward and in the reverse directions.

Referring more particularly to FIG. 1, a combination electrical schematic and perspective view'of'afirst illuse trative embodiment of the present invention is shown connected and utilized to produce nonreciprocal attenuation at the lower radio frequencies normally encountered in lumped constant systems.

As shown in FIG. 1, the isolator is interposed bet-ween a source of radio frequency excitation represented, for

example, by a vacuum tube 10, and an output circuit represented by load impedance i l. The isolator itself comprises a pair of resonant circuits arranged in two meshes. The first, or input mesh which connects to the source of excitation .10 comprises the series-connected resonant circuit including variable capacitor 12. and inductor 13. (A high-impedance radio frequency choke 16 is connected across the input to the isolator merely to provide direct-current continuity from a source of direct-current potential 17 to vacuum tube 10.) The second, or output mesh connects to the load 11. The input and output meshes are mutually coupled by a combination of reciprocal and nonreoi-prooal coupling means. Reciprocal coupling is provided by the second resonant circuit which comprises the shunt-connected variable capacitor 14 and inductor -15. Nonreciprocal coupling betweenthe meshes is provided by means of inductor 13 and 115 and a disc 18 of gyromagnetic material. The term "gy-rom-agnetic material is employed here in its accepted senseas designating the class or magnetically polarizable materials having unpaired spin systems involving portions of the atoms thereof that are capable of being aligned by an external magnetic polarizing field and which exhibit a significant precessional motion at a frequency within the range contemplated by the invention under the combined influence of said polarizing held and an orthogonally directed varying magnetic field component. This precessional motion is characterized as having an angular momentum and a magnetic moment. Typical of such materials are ionized gases, paramagnetic materials and ferromagnetic materials, the latter including the spinels such as magnesium aluminum ferrite, aluminum zinc ferrite and the rare earth iron oxides having a garnet-like structure of the formula A B O where O is oxygen, A is at least one element selected from the group consisting of yttrium and the rare earths having an atomic number between 62 and 7t inclusive, and B is iron optionally containing at least one element selected from the group consisting of gallium, aluminum, scandium, indium and chromium. In the particular embodiment of the invention shown in FIG. 1, a disc of aluminum-substituted yttrium iron oxide is used.

In particular, the coils 13 and '15 are wound on a common core 1 8 of 'gyromagnetic material so that their axes, and consequently their magnetic fields, intersect at right angles to each other. So wound, there is substant-ially no direct coupling between the two coils as a result of their physical relationship to each other. In accordance with the invention, the core material is loosely coupled to the coils, the coupling being in the range of critical coupling or below. As such, the degree of coupling between the coils which results as a consequence of the presence of core 18 is small compared to the reciprocal coupling and for the purposes of the invention maybe neglected.

The core 18 is n the shape of a disc with its faces parallel to the plane of the axes of cores 13 and 15. i A static magnetic field H is applied normal to the faces of the disc and is adjusted to produce gyromagnetic resonance at the frequency of interest.

The biasing field may be supplied by any suitable means (not shown) such as an electric solenoid, a permanent magnetic structure or, in some instances, the core 18 may be permanently magnetized.

To produce isolator action, conditions must be established whereby energy can be dissipated in one direction of transmission to a substantially smaller degree than in the reverse direction of transmission. In the isolators constructed in accordance with the invention, the phenomenon of gyromagnetic resonance is utilized to provide the necessary loss mechanism. At is well known, magnetically polarized gyromagnetic materials exhibit distinctly different properties depending upon the nature of the applied magnetic fields. These unusual properties which are produced can be explained by recognizing that the gyrornagnetic materials contain unpaired electron or nuclear spins which tend to align themselves with the polarizing field but which can be made to process about an axis parallel to the direction of this field by the application of a high-frequency magnetic field. The magnetic moments associated with the spinning atomic particles, however, tend to precess in only one angular sense and will resist rotation in the opposite sense. It is therefore evident that oppositely circularly polarized waves influence the gyrornagnetic material differently, depending upon their sense of rotation. This is so since a circularly polarized wave rotating in one direction will be rotating in the easy angular direction of precession of the magnetic moments whereas an oppositely rotating circular polarized wave will be rotating in a sense inconsistent with the natural behavior of the magnetic moments of the gyromagnetic material. As a consequence, when the highfrequency magnetic field is rotating in the same sense as the preferred direction for precession of the magnetic moments, it couples strongly to the gyromagnetic material. However, very little coupling takes place between the external magnetic field and the magnetic moments when the high-frequency magnetic intensity is rotating in the opposite angular direction.

While this difference in coupling, and consequent difference in permeability provided by oppositely rotating circularly polarized magnetic fields is not limited to any particular frequency or polarizing field strength, particularly useful effects are observed at gyromagnetic resonance when the frequency of the circularly polarized magnetic field is the same as the natural precessional frequency of the magnetic moments as determined by the strength of the polarizing field. Under these particular conditions, a large amount of power can be extracted from a magnetic field circularly polarized in the preferred sense and absorbed in the gyromagnetic material. HOW- ever, very little power is absorbed from an oppositely circularly polarized component.

It is apparent, therefore, that a circularly polarized magnetic field must be generated whose sense of rotation is dependent upon the direction of propagation of the signal through the network.

FIG. 2 is an electrical schematic representation of the embodiment of the invention shown in FIG. 1, whichisineluded to demonstrate how the necessary circularly polarized magnetic field conditions are realized. In this schematic representation, the corresponding reference numerals used in FIG. 1 are also used in FIG. 2whenever possible.

If the operation of the circuit is thought of in terms of a perturbation phenomenon, the device may be considered as comprising an input and an output branch coupled by means of a pair of electrically independent coupling paths. The first coupling path includes a reciprocal coupling means for exciting coils 13 and 15 ninety degrees out of time phase. The second coupling path includes the nonreciprocal loss mechanism and comprises the gyromagnetic material 18 and coils 13 and 15. For the sake of simplicity, in FIG. 2, certain changes and omissions have been made. For example, the vacuum tube 10 has been replaced by a generator .25 and its equivalent internal resistance 26. The effects produced by the slight mutual coupling between coils 13 and 15,

due to the presence of the. gyromagnetic core material 18, has been omitted as has been the radio frequency choke 16. Similarly, the load 1-1 is shown as .a resistive impedance since any reactive component of the load may be tuned by means of capacitor 14.

' Before proceeding with a detailed discussion of the operation of the isolator, it is well to define, in a general way, the function of the several portions of the network shown in the embodiment of FIG. 1. Briefly, it is a function of the reciprocal coupling path to couple the input mesh to the output mesh and in cooperation withthe other components in the network (such as the tuning capacitors) to establish the required time phase relationship between the currents in the second, or nonreciprocal coupling path. It is then the function of the nonreciprocal coupling acting upon the gyromagnetic material to produce the nonreciprocal loss effects characteristic of an isolator. If the coupling between the circuits and the gyromagnetic material is loose, as has been. previously indicated, the operation of the circuit may be thought of in terms of a perturbation phenomenon and, as a first approximation, the interaction between the coupling paths may be neglected. Thus, in the following discussion, the two coupling functions are considered separately and independently.

Let us now consider the operation of the system. Under the influence of generator 25, a current 20 is caused to flow. With both tuned circuits adjusted to resonance, this current is in time phase with the applied voltage E as shown in the time vector diagram in FIG. 3. The component of current 21 flowing in inductor 15, however, is in time quadrature with current 20 and is so shown in FIG. 3. The effect of these two currents is to produce the two magnetic field components gf and f shown in the space vector diagram of FIG. 4. Because of the spatial orientation of the coils, these field components are in space quadrature as well as time quadrature and are so shown.

Because of the ninety degree times difference between current 20 and current 21, as current 20 in coil 13 passes through its maximum amplitude and starts to decrease towards zero, current 21 in coil 15 is passing through zero and is starting to increase towards its. maximum value. Correspondingly, the magnetic fields f and f produced by the currents and 21 respectively, are likewise varying in amplitude in a similar manner. The effect of having the field components f and f varying in this manner is to produce a single resultant field vector which appears to rotate in space in the region of the gyromagnetic material 13. With the polarizing field H directed normal to the plane of field components i and r as shown in FIG. 4, a negatively or counterclockwise rotation of the resultant magnetic field is produced when viewed along the direction of the biasing field. This sense of circularly polarized magnetic field, however, is oppositeto the natural precessional sense of the magnetic moments in the gyromagnetic material and little or no interaction takes place between the electrical energy and the core material and. consequently substantially all the energy is delivered to the load resistor 11.

However, for energy transmitted in the reverse direction, the'sense of rotation of the resultant magnetic field is also reversed. This may be shown by considering the effect upon the circuit when the output mesh is energized by a voltage E (not shown) in series with load resistor 11. E is merely representative of adisturbance in the circuit of an undesirable nature. The resulting current 28 caused to flow by E is in time phase with E and is so represented in the time vector diagram in FIG. 5. The component of current 22 flowing through inductor 13 is also in time phase with E whereas the com ponent of current 23 in inductor 15 is in time quadrature with E The spatial orientation of the magnetic fields produced by these currents is shown in FIG. 6. As in the case of transmission in the forward direction, the field components and fz are at right angles to each other. However, for the reverse transmission case, the direction of the current flow 22 in coil 13 is reverse with respect to current 20 and hence the field fzz is 180 out of phase with field fila- As a consequence, the resultant field producted by fields and i appears to rotate in a positive or clockwise sense as viewed along the direction of the biasing fields H This sense of rotation is the same as the preferred sense for precession of the magnetic moments in the core material'and hence energy is absorbed from the circuit and dissipated in the core material.

In a second embodiment of the invention shown in FIG. 7, the reciprocal and the nonreciprocal coupling between the input and output meshes have been separately provided for. The reciprocal coupling, which establishes the necessary time phase relationship between the energizing currents, is provided by a transformer comprising coils 70 and 71. The nonreciprocal coupling, in turn, is provided by coils 72 and 73 and gyromagnetic material 81.

The input mesh is energized by means of generator 74 which has an output impedance 75. Variable capacitor 76 is adjusted to tune the input mesh. The output mesh provides power to the output load 77, and is likewise tuned to resonance by means of the variable capacitor 78.

As a result of the excitation provided by generator 74, a current 75 flows through coil 70 and coil 72. With the output mesh tuned to resonance, the induced current 80 lags the primary current 79 by ninety degrees. The relationship between the currents is shown in the time vector diagram of FIG. 8. As in the embodiment shown in FIG. 1, coils 72 and 73 are wound on a common core 81 of gyromagnetic material so that their magnetic fields intersect at right angles to each other. The resulting magnetic flux produced by the coils 72 and 73 under the influence of currents 79 and 80 is circularly polarized and appears to rotate in a counterclockwise sense as viewed in the direction of the biasing field H In the reverse direction, a current 82 induced by some source of excitation (not shown) in the output mesh, induces a current 83 that lags current 82 by ninety degrees. Thus, as can be seen in FIG. 8, for the reverse direction of transmission through the network, the current in coil 72 is reversed. This has the effect of reversing the sense of rotation of the resulting field vector so as to produce a resulting magnetic flux that is circularly polarized and rotating in a clockwise sense as viewed in the direction of the biasing field H As has been indicated previously, clockwise rotation of the magnetic field vector causes coupling to the magnetic precession of the gyromagnetic material and power dissipation.

It had been mentioned previously that the loops 13 and 15 of FIG. 1 and loops 72 and 73 of FIG. 7 are loosely coupled to the gyromagnetic material. The'sig-' nificance of this statement may best be understood by referring to FIG. 9 which is essentially FIG. 7 redrawn, with the nonreciprocal coupling network of loops 72, 73 and core 81 replaced by its equivalent circuit 91.

It may be shown that coils 72 and 73, and element 81 may be represented by the equivalent network 91 which includes the nonreciprocal phase shifter 92 and inductors L L L capacitor C and resistor wL Q. L and L are primarily the self-inductance of loops 72 and 73 with a small addition due to the coupling by the gyromagnetic material. These quantities are relatively small and for the purpose of the following discussion may be neglected. The quantity L is an equivalent inductance proportional to both the loop coupling to the gyromagnetic material and the actual magnetization value of the material. C is the equivalent tuning capacitor which tunes the network to gyromagnetic resonance, and the shunt resistor wL Q is defined by the material Q and the coupling L If the coupling between the loops and the gyromagnctic material is large, L and consequently the shunt resistor wL Q are extremely large and the resistive interaction between the shunt resistor and the rest of the network is small. If, on the other hand, the coupling is small, there is essentially a short circuit across the network at resonance and again there is little interaction between the network and any dissipative component. However, for the range of coupling about critical coupling, the value of the shunt resistance is such as to introduce a substantial dissipative element in the circuit. It remains to be shown, however, that the lossy element wL Q is not involved in transmission in the forward direction, but is for transmission in the reverse direction.

In considering FIGS. 7 and 8, it was shown that when the input mesh was excited by generator 74, the current 80 in coil 73 lagged the current 79 in coil 72 by ninety degrees. These currents are shown in FIG. 9 as current l lo entering network 91 at terminal 1 and apparently leaving by terminal 1' and as current I l-90 entering network 91 at terminal 2 and apparently leaving by terminal 2.

Referring current 80 to the input side of the nonreciprocal phase shifter 92, the current undergoes a ninety degree phase advance and as a consequence is now in time phase with current 79. It is as if current entering at terminal 1 leaves at terminal 2, and current entering at terminal 2 leaves at terminal 1'. In this situation, continuity throughout the circuit is maintained Without the necessity of any current flowing through the lossy element wL Q.

For transmission in the reverse direction, however, the current 83 in the input mesh lags the current 82 in the output mesh by ninety degrees. To be consistent with the vector diagram of FIG. 8, current 82 is designated as fi l-99 and current 83 is designated as I d-180". hot h are shown as dotted lines. Again referring I to the input side of the nonreciprocal phase shifter 92, the current undergoes a 90 phase advance and now has a zero phase angle associated with it. As a consequence of this phase advance, current 82 is 180 out of time phase with current 83. This implies that the direction of flow of current I was incorrectly assumed and that in fact I actually flows in the reverse direction. If the sense of direction of I is reversed as shown in FIG. 9, then the currents I and I are in phase. it is therefore as if the currents I and I enter the nonreciprocal coupler at terminals 1' and 2. respectively, flow towards each other and up through the resistive element wL Q, then separate and flow out through terminals 1 and 2 respectively. Thus, in the reverse direction, the lossy element in the equivalent circuit 9'1 is involved in the transmission path and is a. measure of the attentuation experienced by signals propagating in the reverse direction.

In all preceding discussions, the rotating magnetic field induced in the gyromagnetic material was referred to as being circularly polarized. This assumes that the two orthogonally directed magnetic field components making up the equivalent rotating field are of equal amplitude. In a well-designed isolator, this will be so since any inequality in these two fields gives rise to an equivalent elliptically polarized field. Since an elliptically polarized field may be resolved into oppositely rotating circularly polarized field components, the efiect is to induce losses in the system for transmission in the forward direction and to reduce the attenuation experienced in the reverse direction.

The system therefore is designed to match the load impedance to the generator source 'by means of the reciprocal coupler and to equalize the field components produced by the resulting energizing currents by adjusting the turns ratio of the coils associated with the nonreciprocal coupler.

It is a feature of the invention that changes in the load impedance which would normally react upon the source and in some instances adversely affect the operation of the source, instead tend to unequalize the relative amplitudes or modify the quadrature phase relationship of the orthogonal field components. .This, in turn, tends to induce elliptical polarization in the gyromagnetic material. system, the unbalance in the network does not tend to react upon the source but rather roduces an unbalance in the exciting fields which results in that portion of the energy that would normally be reflected being absorbed in the gyromaignetic material. This loss mechanism produces an inherent stability in the system which tends to isolate the source from any changes in the load as well as from spurious signals originating elsewhere in the system.

In the particular embodiments of the invention shown in FIGS. 1 and 7, circular polarization was induced in the gyromagnetic material by making both the time phase and the space phase of the exciting field components ninety degrees. It should be noted, however, that there are other combinations of time and space phasings which also produce a circularly polarized resultant field. It can be shown that, in general, for any arbitrary space orientation, a, of the two field components (where a is. assumed to be positive) there is a time phase ,8 that produces circular polarization, where [3==(2n+l)ra, n being any whole number. Thus, in other circuit arrangements, and for other applications which more readily lend themselves or which require time phase differences other than 90 degrees, circular polarization may nevertheless be induced by arranging the field components at an angle in accordance with the above equation.

In all cases it is understood that the above-described arrangements are illustrative of a small number of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can readily be devised in accordance with these principles by those skilled in the art Without departing from the spirit and scope of the invention.

What is claimed is:

1. A nonreciprocal passive four-terminal network comprising a pair of inductive coils having substantially mutually perpendicular axes intersecting at a point, first means for exciting said coils substantially ninety degrees out of time phase with respect to each other, second means for exciting said coils to a substantially smaller degree than said first means comprising a magnetically polarizable element exhibiting gyromagnetic effects over the operating frequency of said network located in a region including said point, and means for magnetically biasing said element in a direction normal to the plane of said intersecting axes.

2. In an electromagnetic transmission system means for producing nonreciprocal attenuation comprising a pair of conductive coils, first means for exciting said coils ,8 degrees out of time phase with respect to each other, said coils when excited being supportive of, magnetic fields having components intersecting in a region at an angle a where ;3 equals (2n+l)ira, n being an integer, second means for exciting said coils to a substantially smaller degree than said first means comprising a magnetically polarizable element exhibiting gyromagnetic efiects over the operating frequency of said system located in said region, and means for magnetically polarizing said element in a direction normal tosaid intersecting components.

3. In an electrical transmission system means for producing isolator effects comprising first and second serially connected inductive coils, a first condenser connected in series with said first coil to form a first resonant circuit tuned to a frequency h, a second condenser connected :in shunt with said second coil to form a second resonant circuit tuned to said frequency f the axes of said first As a result of these changes in the coil being substantially perpendicular to the axes of said second coil to substantially preclude coupling therebetween, input means connected across both serially connected coils and said first condenser, output means connected across said second-coil and said second condenser, nonreciprocal coupling means for loosely coupling said coils comprising a magnetically polarizable element exhibiting gyromagnetic effects over a range of frequencies including said frequency f and means for magnetically biasing said material to gyromagnet-ic resonance at said FOREIGN PATENTS frequency h in a direction normal to both of said axes- 715,257 Great Britain Sept, 29, 1 5 References Cited in the file of this patent 1O41549 Germany 1958 UNITED STATES PATENTS 5 QT REFERENCES 2,647,239 Tellegen Jul 28, 1953 fg 1 8 Research Reports Apnl 2,697,759 Tellegen et a1. Dec. 21, 1954 palges t a 4] 1 f A d Ph l 27 2,764,676 Bradley Sept- 5 1 5 lano a e a ourna 0 pp 16 y v0 r 2,944,229 De Vries July 5, 1960 June 1956 pages 608409 

